I think you are referring to citric acid crystals?
To make a 50% water solution of citric acid and distilled water use equal parts of both. They should be weighed, but you can use volume measurements and be close enough.
From Jim Humble's 2011 book:
To Make a 50% Solution
Pack a cup or small bowl with citric acid powder (crystals), tamping it down to fill air holes.
Dump it in a larger bowl. Then fill the same cup or small bowl with distilled or purified water and dump that in the larger bowl.
The citric acid powder weighs almost the same as water and is close enough for the purpose. 75 g each will fill a 4 fl.oz. bottle. Stir until dissolved. You will notice that the mixture becomes cold.
Then pour it into a bottle(s) with a drip style top for counting drops and label it 50% citric acid. Use it for activating MMS at one drop for each drop of MMS.
